I suggest you to learn the concept of assemblies in c#. As Mr.Naveen said its not easy to understand with few lines of answers posted here.

Get the basic idea of dll(dynamic link library) and dig deep to master the concept.

Collection of classes is called as namespace, collection of namespaces is called as assembly.These assemblies(dll's) contain the reusable logic which is developed in one programming language and can be used in another language while developing applications using add reference of that particular dll.